In the United States, approximately 40% of energy consumption is used to heat and cool buildings. In buildings, the majority of energy loss takes place through the building envelope and the HVAC air intake/exhaust system. The building envelope consists of doors/windows, exterior wall systems and roofing system. While great progress has been made improving the energy efficiency of roof, door and window systems, very little progress has been made in designing truly energy-efficient exterior wall systems.
Framed walls use metal or wood studs to build a frame that can be either load bearing or infill. Multistory buildings can be made from cast-in-place concrete with the exterior perimeter walls being in-filled frame construction. Exterior sheathing is attached to the outside of the frame. On the inside, drywall is used for the inside finish surface. This framing system creates a cavity between the exterior sheathing and the drywall. This cavity is then filled with batt insulation to improve energy efficiency. It is assumed that the R-value of the batt insulation determines the energy efficiency of the wall system. However, there are several drawback of this system. Framing members create thermal bridging. Batt insulation may not completely fill the cavity wall and over time it can sag, leaving no insulation in places. Moisture condensation inside the cavity wall is common which dampens and compresses the batt insulation. When this occurs, the damp batt insulation loses most, if not all, insulating properties. HVAC systems create pressure differentials between the interior and the exterior of the building. These pressure differences cause air to move through the exterior wall system. Simply stated, cavity wall framed systems have poor energy efficiency, among many other problems.
Exterior walls can also be made of concrete, either pre-cast or cast-in-place. Concrete is a composite material comprised of a mineral-based hydraulic binder which acts to adhere mineral particulates together in a solid mass; those particulates may consist of coarse aggregate (rock or gravel), fine aggregate (natural sand or crushed fines). While concrete provides a long lifespan and increased protection from damage, concrete is as cold or as hot as the ambient temperature. Concrete has high thermal mass, which makes it rather expensive to heat or cool in extreme temperatures. In an attempt to alleviate this problem, the inside of the building is insulated. However, such insulation does little to improve energy efficiency as it is generally on the wrong side of the wall; i.e., the interior wall surface. Concrete walls have the advantage that they are barrier systems; i.e., no air can flow through from inside to the outside, but still have poor energy efficiency.
Exterior masonry walls are typically made of CMU (concrete masonry unit) blocks or brick. The block wall cavity is filled with concrete for structural enforcement and foam for insulation. However, this does little to improve energy efficiency since concrete thermal bridging surrounds each foam cell. Brick walls have better insulating properties than concrete or CMU block walls, but still have poor energy efficiency.
Exterior Insulated Finish Systems (EIFS) are used as exterior wall cladding to improve the energy efficiency of the building envelope. EIFS is a versatile, cost effective and relatively energy efficient barrier system. However, EIFS also has several disadvantages. EIFS has relatively low impact resistance. Animals, such as woodpeckers, can cause severe damage to EIFS. And, EIFS is not as long lasting as concrete, stucco or brick. Furthermore, when applied over framing systems there is still a wall cavity to contend with, and drainage cavities are required to mitigate some of the issues associated with the use of EIFS, such as water intrusion, mold and others. Moreover, application of EIFS requires scaffolding the entire building perimeter, which adds cost and time to a construction schedule. Due to all of the above, and more, many owners and architects will simply not consider using EIFS on their projects.
To improve performance and reduce construction schedules, exterior framed walls have been panelized. The exterior building envelope is divided up in small enough panels that can be framed at a plant. These metal framed panels are then sheathed on the exterior face and EIFS, stucco or thin brick is installed over the sheathing. The panels are thus finished and shipped to the project site ready to be erected in place. Steel embeds and connections are used to attached the framed panels to the building structure. While this system improves construction schedules and eliminates the need for exterior scaffolding, but it still includes an exterior wall cavity formed by the framing members, with all its associated shortfalls.
Precast or structural concrete wall panels are known in the art. Precast infill concrete panels are used for non-load bearing purposes. The use of precast concrete wall panels has gained in popularity because they can be manufactured at a remote location, transported to a job site and attached into place, usually be welding steel embeds to a building's structural frame. Precast structural panels can also be formed both onsite and offsite and used to support a load bearing structure of one to four stories tall.
Prior art precast concrete wall panels also have a large thermal mass when exposed to ambient temperatures. They retain the heat in the summer or the cold in the winter very well. Therefore, buildings built with precast concrete panels generally have relatively poor energy efficiency. Such buildings usually require a relatively large amount of energy to keep them warm in the winter and cool in the summer. Since most precast concrete panels are not insulated, they must be insulated on the inside through the use of interior framing systems. This method however does not create a highly energy efficient building envelope. And, since batt insulation of significant thickness is required the interior frame system takes valuable floor space and creates a cavity wall.
More recently, new methods of insulating precast concrete panels have been employed. There are a number of insulated concrete panel systems currently employed. All of them are a “sandwich” type panel. Such panels require placing a layer of foam between two layers of concrete. Some panels are non-composite while others are composite types. While this method improves the insulating properties of a wall and therefore the energy efficiency of a building, it has several drawbacks.
One method involves placing a layer of insulation between a structural concrete layer and an architectural or non-structural concrete layer during the casting of the panel and then erecting this entire non-composite construction as an exterior panel. While this method improves the insulating properties of a wall and therefore the energy efficiency of a building, it has several drawbacks. Instead of having one layer of concrete, the “sandwich” creates two; one that is structural with the larger thermal mass that faces the inside of the building and is insulated from the elements. The second layer of concrete is slightly thinner and placed on the exterior of the building; i.e., on side of the panel opposite the insulated structural layer. Although the second layer is thinner than the first layer, it usually includes steel reinforcing bars (“rebar”). Rebar has to have a minimum embedment of 1½ inches from the exterior face of concrete and is usually placed in the center of the concrete. Therefore, the thinnest exterior concrete is still approximately 3 to 4 inches thick. The second layer is therefore still relatively thick and heavy. The weight of the second layer added to the weight of the first layer makes the entire panel relatively heavy. The American Concrete Institute and industry practice requires that no shear forces be exerted by the first and second layers of the “sandwich” on the insulating layer. Therefore, a bond breaking layer is applied to the insulating layer so that neither the first nor the second layer will adhere thereto. Since there is no bond between the two layers of concrete and the foam, the ties used to connect the two concrete layers have to be engineered to resist the shear pressure from the weight of the second layer of concrete. Generally this is a costly system.
Other methods of sandwich panel construction involve a layer of foam between two wythes (layers) of concrete in a composite type assembly. The inner and outer wythes can be the same thickness or the inner wythe can be thicker while the outer wythe can be thinner. Some use composite plastic ties to hold the two wythes together while others use carbon fiber mesh. Some sandwich panels use pre-stressed cables to achieve the required strength while others use internal trusses. However these panels are heavier and therefore more expensive to manufacture. Since the exterior wythes are made from conventional concrete, they are still considerably thick due to minimum steel embedment code requirements. The thinner the concrete wythes, the more brittle they become which requires use of pre-stressed cable reinforcement or expensive carbon fiber reinforcements. To place the steel embedments, attachments and reinforcement, the thinnest practical concrete thickness is limited to approximately 2 to 3 inches.
Therefore, it would be desirable to provide a system for relatively easily and efficiently insulating precast concrete panels or other structures to achieve the highest energy efficiency possible. It would also be desirable to provide a precast concrete panel system that provides a relatively lightweight precast infill cementitious-based or cementitious panel. It would also be desirable to provide a composite precast insulated panel that is lighter, thinner and stronger than prior art panels. It would also be desirable to provide an integrated architectural finished composite precast insulated panel that can incorporate various types of finish textures, colors, and patterns such as concrete, plaster, stucco, stone, brick, tile and the like.